* Machine addresses represent real RAM in the host. The memory a guest owns will certainly not start at 0 and will not necessarily be contiguous - it might be in a number of chunks with big gaps between.

* (pseudo)physical addresses represent the memory the guest owns. This address space starts at 0 and is contiguous.

* Virtual addresses are used by software running in the guest, and by the guest kernel. They're translated by the host CPU into machine addresses so that it can access the correct RAM.

Guests use physical addresses as an abstraction: most operating system memory management code assumes that the RAM owned by the OS starts at 0 and is contiguous. Because this is not the case for Machine addresses under Xen, most of the guest's code is "tricked" by giving it pseudophysical addresses that look like it expects memory to look.

The P2M and M2P tables record the relationship between pseudophysical page frames (which the core OS code uses) and machine page frames (which the host really uses). The Xen "architecture" code within the guest OS uses these tables to manage the translation between pseudophysical and machine page frames so that the guest's page tables can be handled correctly. For paravirtualised guests, page tables must contain machine addresses - these must be translated from the pseudophysical addresses used by core OS code.

Hope that helps clarify how this all fits together, tgh.

Cheers, Mark